The present invention relates to a method of forming a single crystal semiconductor layer from a non-single-crystalline material, and an apparatus for forming the same. More particularly, it relates to a method of forming a single crystal semiconductor layer from a non-single-crystalline material by scanning the surface of the material with an electron beam in a first direction while rapidly deflecting the beam in a second direction perpendicular to the first direction, and an apparatus for forming the same by scanning the surface of the material with an electron beam in this manner.
Recently, various methods of forming SOI (silicon on insulator) layers, using electron beam annealing, have been developed to be used in the semiconductor industry. These methods comprise the steps of forming an insulative layer of silicon dioxide or silicon nitride on a single crystal silicon substrate, depositing a non-single-crystalline layer such as a polycrystalline or amorphous silicon layer on the insulative layer, and applying an electron beam or a laser beam to the polycrystalline or amorphous silicon layer, thus annealing this layer and recrystallizing the silicon, and forming a layer of single crystal silicon.
In the known method described above, a converged electron beam having Gaussian distribution scans a surface region of a non-single-crystalline layer in the X and Y directions, thereby annealing this region. The electron beam used in this method has a diameter of 10-500 .mu.m. Therefore, the surface region, which is melted as the beam scans the substrate along a line, has a width equal to about the diameter of the beam. A crystal grain bundary is inevitably formed in a surface region on which the scanning lines are overlapped. In view of this, the method is not suitable for forming a relative large area of single crystal silicon.
Very recently, a new method of forming an SOI layer has been invented. In this new method, such a sine wave voltage having a relatively high frequency as is shown in FIG. 1B is applied to deflection electrodes, which rapidly deflect an electron beam in the Y direction to produce a so called pseudo-line-shaped beam, thus forming a straight locus of the beam spot on the substrate, while the beam is scanning a substrate in the X direction, as shown in FIG. 1A. This method is regarded as promising since the length of the straight locus of the beam spot is determined by the deflection angle, and theoretically there is no limit to the length of this locus. However, the greater the deflection angle, the lower the surface temperature of the scanned substrate, as shown in FIG. 2. The surface region of the semiconductor substrate must be sufficiently heated to form a semiconductor, single crystal layer. Therefore, when the electron beam is deflected at a large angle, a greater current must be supplied to an electron gun so that the gun may emit a more intense electron beam. Here arises a problem. The electron gun has its own luminance characteristic. The intensity of the electron beam cannot increase, no matter how great the current supplied to the gun. Hence, the maximum length of the straight locus of the beam spot is limited by the luminance characteristic of the electron gun.
The surface temperature distribution of the surface of the non-single-crystalline material scanned by the method described above is not uniform along the straight locus of the beam. Moreover, the intensity distribution of the beam on the surface of the material is not uniform since the sine wave voltage (FIG. 1B) is applied to the deflection electrodes. FIG. 3 shows the surface temperature distribution of a non-single-crystalline semiconductor material, along the straight locus of the beam spot. As this figure shows, two portions of the semiconductor material, near the ends of the locus of the beam spot, are heated to a temperature higher than any other portion. When these two portions are properly melted, the portion at the middle of the straight locus is insufficiently melted. As a result, the surface of the semiconductor material is not uniformly annealed.
To avoid such a non-uniform melting, a triangular wave signal, not a sine wave signal, may be applied to the deflection electrodes. This is because, the use of a triangular wave signal may make the probability of the beam spot staying at a point on the semiconductor material, uniformly distributed along the straight locus of the beam spot. This method is ineffective when the frequency of the deflection voltage is relatively high. The higher the frequency of the signal, the more the triangular waveform is distorted to become similar to a sine wave. Consequently, said probability varies along the straight locus of the electron beam spot. The deflection signal must have a frequency in the order of MHz. As shown in FIG. 4, the higher the frequency of the deflection signal, the less the probability varies along the locus, and the less the surface temperature changes along the straight locus of the beam spot. The change of the probability is negligibly small when a signal having a frequency of about 2 MHz or more is applied to the deflection electrodes.
As mentioned above, various drawbacks are inherent in the known method, wherein a sine wave signal or a triangular wave signal is applied to the deflection electrodes to rapidly deflect a converged electron beam in the Y direction while scanning a non-single-crystalline material in the X direction. It is difficult in this method to form a semiconductor, single crystal layer of an even composition.